Go buy some clove oil, I would be willing to bet $100 that anyone after me suggests the same thing. I have personally not used it, but I have seen a few threads on here just like this one and it is what everyone recommends.
Major bummer to lose your fish, especially one you have had for so long. Doesn't take but a month to feel attached to a fish, I cant imagine how it will be once I have had mine for 4-5 years. But really, clove oil will put the fish to sleep, then basically it just goes all numb I guess and passes on. I don't know how much you have to use or whatever, but you put the fish in a bucket, put in a bunch of clove oil and the dead is done quick and painless.
If for any reason you didn't want to go with clove oil, I have also heard of putting the fish in a bucket of ice water. If you where going to do this I would fill a bowl or something with enough water to hold the fish, like just enough, then fill the rest with ice and throw the whole thing in the freezer for awhile to seriously drop the temp. After maybe 30 minutes in the freezer the water will be so cold that if you dropped the fish in it would die instantly and feel no pain I would think. It would just shock its heart so fast it would not even know what happened. Most people just suggest ice water, but I don't think this would be cold enough to do it quick enough. I would want the water as cold as possible.
Again, sorry to hear about your fish. I hope this helps it pass in the most humane and painless way possible.